Dc electric motor with asymmetrical stator inductors

ABSTRACT

A DC motor including a continuous rotation rotor; a first inductor characterized by first parameters; a second inductor characterized by second parameters; a voltage supply unit; a measurement unit for detecting time instants when a first induced voltage in the first inductor equals a second induced voltage in the second inductor; and a control unit for controlling the application of drive voltage pulses to the inductors. The rotor faces first the second inductor before facing the first inductor when being rotated. At least one of the second parameters is selected different from a corresponding parameter of the first parameters such that a maximum induced voltage in the first inductor is greater than a maximum induced voltage in the second inductor. The control unit is arranged to trigger each of the drive voltage pulses after a detection of an equal induced voltage in the first and second inductors.

TECHNICAL FIELD

The present invention relates to a direct current (DC) electric motorcomprising two circular coils of relatively small thickness (i.e.disc-shaped coils) as stator inductors. The proposed motor may havesmall dimensions making it suitable for horological applications andmore specifically for electromechanical watch movements.

BACKGROUND OF THE INVENTION

DC electric motors are well known and have been around for quite a longtime. These motors convert electrical energy into mechanical energy formany types of applications. Electromechanical mobile devices, such aselectromechanical wristwatches, often comprise a DC motor and arepowered by DC sources, such as batteries. One example of a DC motor is apermanent magnet DC motor. This kind of DC motor has no brushes and hastypically permanent magnets on the rotor. The stator comprises coils,which typically do not move. This kind of electric motor allows forsmaller design and results in reduced power consumption.

In horological applications, stepper motors are generally used. Specificcommands generate drive electrical pulses, which make the rotor advancestep by step. Stepper motors are brushless DC motors, which divide afull rotation into a number of equal steps. The stator defines stablepositions for the rotor with permanent magnets. There are typically twoor three stable positions per one full rotation of 360 degrees. To beable to generate the pulses, a sufficient voltage level is needed.Voltage supplies used in these motors, especially when used inelectromechanical watches, typically generate a voltage level between1.2 V and 1.5 V. Consequently, batteries available for theseapplications supply a voltage in this range of values. Continuousrotation DC electric motors have the advantage over stepper motors thatwhen used in horological applications, the watch hands can be rotatedcontinuously. This makes the operation of these watches similar tomechanical watches. In this manner noise caused by the steps of therotor, which could be disturbing in particular at night time, can beavoided.

A DC motor is controlled by a motor drive unit. The drive units aretypically arranged to alternate the current that travels in the statorcoils and thus the direction of the magnetic flux lines which arecoupled to the magnet(s) of the rotor. An H-bridge circuit is an exampleimplementation of a motor drive unit. The term H-bridge is derived fromthe typical graphical representation of this kind of circuit comprisingfour switches arranged between a supply voltage node and ground. Byopening and closing these switches in a desired manner, a positive ornegative voltage can be selectively applied to the inductor circuit ofthe motor. In other words, by manipulating the four switches dependingon the position of the rotor or more specifically on the rotor magnets,a current can be arranged to travel through the stator coils selectivelyin a first direction, and in a second, opposite direction.

An example DC motor arrangement is schematically illustrated in FIG. 1.The simplified motor 1 of FIG. 1 comprises a rotor 3, with permanentbipolar magnets 3 b arranged on two ferromagnetic discs 3 a (thesemagnets having an axial polarisation and alternate polarities), and astator formed by a first stator inductor A and a second stator inductorB. A motor drive or control unit 5 is configured to adjust the currentthrough the coils. A digital control unit 7 is in turn configured tocontrol the operation of the motor drive unit based on a detectedoperation of the rotor. For instance, if the control unit 7 detects thatthe rotor is spinning too fast, it can order the motor drive unit 5 toslow down the rotor 3. The motor drive unit is also provided with avoltage supply unit 9, such as a battery. There is further shown ameasurement unit 11 for taking measurements relating to the operation ofthe motor.

SUMMARY OF THE INVENTION

Joule heating, also known as Ohmic or resistive heating, is the processby which the passage of an electric current through a conductor producesheat. Joule's first law states that the power of heating P_(j) generatedby an electrical conductor is proportional to the product of itsresistance R and the square of the current I: P_(j)=R×I². However, theuseful mechanical power P_(mec) is proportional to the current, but notto its square: P_(mec)=k_(u)×w×I, where k_(u) is the torque constant andw the rotational speed of the rotor. Thus, it becomes clear that inorder to minimise the resistive heating losses, the supply currentshould be kept as low as possible while however keeping the motor torquesufficiently high for driving the motor, and more specifically itsrotor.

The resistive heating losses can be reduced if a voltage pulse generatedby the motor drive unit for the stator coils is generated when the sumof the induced voltages in the stator coils is at its maximum value. By‘induced voltage in/of a coil or across an inductor circuit’ it isunderstood the induced voltage (caused by the rotation or turning of therotor) between the two terminals of the coil or of the inductor circuit.Assuming that the voltage supply, provided to the two stator coilsarranged in series, is sufficient for driving the rotor at its nominalspeed with a useful voltage corresponding to the difference between thevoltage supply and the maximum value of the sum of the induced voltagesin the two stator coils, then the ideal situation for driving a motorhaving two identical coils (i.e. having same parameters, in particularsame dimensions and arranged at an equal distance from the rotation axisof the rotor, as would be selected by a person skilled in the art) isillustrated in FIG. 2. The thick solid line indicates the inducedvoltage in the first inductor A and the thin solid line indicates theinduced voltage in the second inductor B. The dashed line indicates thesum of the induced voltages in the first and second coils A, B, whilethe dashed step shape indicates the drive voltage pulses generated bythe motor drive unit. As shown, the ideal situation occurs when eachdrive voltage pulse is centred at an absolute maximum of the sum of thetwo induced voltages. However, it is not easy with low consumptioncontrol circuits to obtain such optimal drive pulses.

A preferred control method of a motor of the type of FIG. 1 consists,within the frame of the present invention, in detecting the crossing ofthe two induced voltages and in triggering the drive voltage pulses bythis crossing detection. With such a preferred control method, thegenerated drive voltage pulses are no longer centred at absolute maximaof the sum of the induced voltages, as illustrated in FIG. 3. Comparedto the situation of FIG. 2, the difference between the supply voltageand the average of the sum of the induced voltage over the duration of adrive voltage pulse is no longer minimal in the situation of FIG. 3,what increases the power consumption of the motor relatively to the caseof FIG. 2 because of the resistive heating losses which are greater.This is thus a non-optimal situation.

It is an object of the present invention to overcome the problemidentified above for a motor of the type shown in FIG. 1 and controlledaccording to the preferred control method described before.

According to a first aspect of the invention, there is provided a directcurrent electric motor comprising:

-   -   a rotor equipped with permanent magnets, the rotor being        arranged to rotate continuously in a determined rotation        direction;    -   a first stator inductor characterised by first inductor        parameters;    -   a second stator inductor characterised by second inductor        parameters;    -   a voltage supply unit for providing a voltage supply to the        first and second stator inductors for driving the rotor;    -   an measurement unit for detecting time instants when a first        induced voltage in the first stator inductor equals a second        induced voltage in the second stator inductor;    -   a control unit for controlling the application of drive voltage        pulses by the voltage supply unit to the first and second stator        inductors,        wherein the rotor is arranged to first face the second stator        inductor before facing the first stator inductor when being        rotated in the determined rotation direction,        wherein at least one of the second inductor parameters is        different from a corresponding parameter of the first inductor        parameters such that a maximum induced voltage in the first        stator inductor is greater than a maximum induced voltage in the        second stator inductor, and wherein the control unit is arranged        to trigger each drive voltage pulse after a detection, by the        measurement unit, of an equal induced voltage in each of the        first and second stator inductors.

According to an advantageous variant, the control unit is arranged totrigger the drive voltage pulses immediately after the detection of anequal induced voltage (i.e. detection of a crossing of the inducedvoltages in the two stator inductors).

According to an advantageous variant, the at least one of the secondinductor parameters comprises at least one structural dimension of thesecond inductor.

The proposed solution has the advantage that the resistive heatinglosses can be minimised because the crossing point of the two inducedvoltages is located before the peak of the sum of these two inducedvoltages and thus the generated pulses are optimally located. In otherwords, the overall power consumption of the motor is minimised withouthowever compromising the motor performance.

According to a second aspect of the invention, there is provided anelectromechanical watch comprising the motor according to the firstaspect of the present invention. According to a third aspect of theinvention, there is provided a method of operating a DC electric motoras recited in claim 12.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description of a non-limiting example embodiment, withreference to the appended drawings, in which:

FIG. 1 illustrates in a simplified and schematic manner a DC electricmotor where the teachings of the present invention may be applied;

FIG. 2 shows the waveforms of the induced voltages in the inductors ofFIG. 1 assuming that these inductors have substantially identicalparameters and further shows an ideal location of the drive voltagepulses in this scenario;

FIG. 3 shows the waveforms of the induced voltages in the inductors ofFIG. 1 assuming that these inductors have substantially identicalparameters and further shows a non-optimal location of the drive voltagepulses resulting from a preferred control method for the motor;

FIG. 4 shows the waveforms of the induced voltages in the inductors ofFIG. 1 assuming that these inductors are asymmetrical (i.e. notidentical parameters) and further shows, according to the invention, anoptimal location of the drive voltage pulses with a preferred controlmethod for the motor;

FIG. 5 shows a schematic rotor-stator configuration according to thesolution of FIG. 2 or 3;

FIG. 6 shows a schematic rotor-stator configuration according to anexample of the present invention;

FIG. 7 schematically shows an example practical implementation of arotor-stator configuration according to FIG. 6; and

FIG. 8 shows the curve of a sum of induced voltages averaged over thepulse duration as a function of the radius of a first inductor andfurther shows the curve of a maximum of the sum of these inducedvoltages as a function of the radius of the first inductor.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described in detailwith reference to the attached figures. The invention will be describedin the context of a continuous rotation DC electric motor of a watch,such as a wristwatch, comprising two stator inductors, where the rotorof the motor is equipped with bipolar permanent magnets. However, theteachings of the invention are not limited to this environment orapplication. Identical or corresponding functional and structuralelements which appear in different drawings are assigned the samereference numerals. As utilised herein, “and/or” means any one or moreof the items in the list joined by “and/or”. The word “comprise” isinterpreted by the broader meaning “include” or “contain”.

The teachings of the present invention are next explained in more detailin the context of the motor of FIG. 1. The rotor is arranged tocontinuously rotate in a first direction but optionally also in asecond, opposite direction. In the present example, the first and secondstator inductors or coils A, B have a disc-shaped (circular) form with acentral through hole (with no magnetic material in it). When used inwristwatches, an external diameter of each coil may be between 3 mm and5 mm, while an internal diameter (of the central hole) may be between0.5 mm and 1.5 mm. The internal diameter corresponds thus to thediameter of the central aperture. The inductors are in this examplerelatively flat and disc-shaped and have a thickness between 0.3 mm and1 mm. The first and second inductors are at an angle ∝ relative to eachother. The angle ∝ is here defined as the angle between a firstimaginary line, passing through the axis of rotation of the rotor andthe axis of rotation of the first inductor, and a second imaginary linepassing through the axis of rotation of the rotor and the centre of thesecond inductor. In this example the angle ∝ is preferably 104° but itcould instead be in an advantageous variant any value between 95° and115° or more specifically between 100° and 110°. In the presentembodiment, the rotor is equipped with six permanent magnets 3 b, havingan axial polarisation axis and alternate polarities, on each of its twoferromagnetic plates 3 a and so that the angle ∝ is 104°. This leads toan electrical phase difference of 48° between the induced voltages inthe inductors.

As explained above, the present invention aims to centre drive voltagepulses time wise substantially at the respective centres of absolutemaxima of the sum of the induced voltages in the two motor statorinductors by keeping the preferred motor control method characterized byproviding drive voltage pulses each after a detection of a crossing ofthe induced voltages in the two inductors. In practical terms, it issearched the minimum between the supply voltage and the sum of the twoinduced voltages averaged over the pulse duration: min(V_(bat)−V _(ind)^(tot)). This can be achieved by having asymmetrical induced voltages inthe first and second inductors A, B such that each drive voltage pulsecan be centred at an absolute maximum of the sum of the inducedvoltages. By ‘absolute maximum’, it is understood a maximum in absolutevalue. This ideal scenario is depicted in FIG. 4. In this scenario, thepeak induced voltage value in the second inductor B (thin solid line) islower than the peak induced voltage value in the first inductor A (thicksolid line) so that a crossing point of the induced voltages is locatedbefore (in the time domain) an absolute maximum of the sum of theinduced voltages. Advantageously the beginning of each drive voltagepulse is located substantially immediately or a very short time durationafter a crossing point. This time duration corresponds to the delayrequired to change the state of the switches of the motor drive unit 5.This delay may be between 400 μs and 800 μs. It is to be noted that thecrossing of the induced voltages in the two stator inductors/coils canbe detected by a comparator arranged within the measurement unit 11.

To achieve the asymmetrical induced voltages, the present inventionproposes a solution in which the two inductors A and B are no longersymmetrical or identical. In other words, the two inductors A and B areasymmetrical with respect to at least one inductor parameter. FIG. 5schematically illustrates a rotor-stator configuration comprising twoidentical stator inductors. This configuration may be considered to beideal when the two inductors are identical. However, this situationleads to a situation depicted in FIG. 3, in which the generated voltagepulse is not centred at the maximum of the sum of the induced voltagesin the first and second inductors. The following notations are used:

-   -   D_(A) denotes the distance between the centre of the first        inductor A and the centre of the rotor;    -   D_(B) denotes the distance between the centre of the second        inductor B and the centre of the rotor;    -   D_(AP) denotes the smallest distance between the periphery of        the first inductor A and the centre of the rotor;    -   D_(BP) denotes the smallest distance between the periphery of        the second inductor B and the centre of the rotor;    -   D_(AB) denotes the smallest distance between the peripheries of        the first and second inductors A, B;    -   R_(A) denotes the radius of the first inductor A; and    -   R_(B) denotes the radius of the first inductor B.

In order to reduce power consumption and thus to approach the idealsituation, in which the pulse is centred at the absolute maximum of thesum of the induced voltages, according to the present invention at leastone inductor parameter of at least one of the inductors is modifiedcompared to the situation shown in FIG. 5 which defines a starting pointfor the configuration of the two coils according to the invention. Thefirst inductor A is characterised by first inductor parameters, whilethe second inductor B is characterised by second inductor parameters.Thus, according to the present invention at least one of the secondinductor parameters is different from at least one correspondingparameter of the first inductor parameters such that in a given rotationdirection of the rotor, the rotor faces first the second inductor beforefacing the first inductor and such that the maximum induced voltage inthe first inductor is larger than the maximum induced voltage in thesecond inductor. The measurement unit 11 is arranged to detect timeinstants when the induced voltage in the first inductor A equals theinduced voltage in the second inductor B and to instruct the drive unit5 accordingly. The drive unit 5 is arranged to trigger the voltagepulses after the detection, by the measurement unit, of an equal inducedvoltage in each of the first and second inductors A, B.

The first and second inductor parameters comprise at least one of thefollowing parameters: coil wire diameter, number of wire turns, coilthickness, coil external diameter and coil internal diameter (or thecentral hole), as well as and the coil position (distance of its centreor periphery from the rotation axis of the rotor). It is to be notedthat the inductor diameters may or may not be constant throughout thethickness of the inductor. If the coil's cross section is notsubstantially circular, the diameter could be replaced for instance witha largest cross-sectional dimension. Thus, in view of the above, atleast one parameter, which is according to the invention differentbetween the first inductor A and the second inductor B, may be a givenstructural dimension of the inductors.

FIG. 6 illustrates a rotor-stator configuration, in which the externalradius R_(A) of the first inductor A has been made greater compared tothe situation of FIG. 5. Furthermore, the external radius R_(B) of thesecond inductor B has been made smaller, the first inductor A has beenrepositioned (the centre moved away from the rotor) and the secondinductor B has also been repositioned (the centre moved closer to therotor) in order to keep the parameters D_(AB), D_(AP), D_(BP) and ∝substantially constant between the configurations of FIGS. 5 and 6.However, it is not necessary to keep all the parameters constant whenmoving from the configuration of FIG. 5 towards an optimal situationaccording to the invention. In one example configuration, D_(AB)substantially equals D_(BP), which equals D_(AP). In view of the above,if one inductor parameter of one of the inductors is altered, then thismay lead to modifying at least one parameter of the other inductor. Inone example, only one inductor parameter of one of the inductors ismodified, namely one of the distances D_(A) or D_(B), when moving fromthe configuration of FIG. 5 towards the ideal situation. This means thatthe angle ∝ may be kept constant when moving from the configuration ofFIG. 5 towards an ideal situation.

However, when modifying at least one inductor parameter of the first orsecond inductor(s), certain tolerances have to be respected. Thesetolerances are due to the practical implementation of the motor 1 asillustrated in FIG. 7. As shown in FIG. 7, a pinion 13 is provided atthe centre of the rotor and occupies a first surface area (circularsurface area) with a given diameter. The pinion is arranged to mesh witha wheel 15, which occupies a second surface area which is a givenangular section of the surface area of the rotor (when seen from above).Thus, under no circumstances are the first and second inductors A, Ballowed to penetrate the first and second surface areas. This means thatthe values of the parameters D_(AP) and D_(BP) are greater than theradius of the pinion and that the angle ∝ between the two inductors mayhave a maximal allowed value. Furthermore, a certain minimum distancemay be defined between the first and second inductors A, B. Thus, inview of the above, it is possible to set minimum distances between therotor and the first inductor on the one hand, and between the rotor andthe second inductor on the other hand. Furthermore, the angle ∝ betweenthe two inductors may have a maximal allowed value.

The upper (thin) curve in FIG. 8 shows the maximum of the sum of theinduced voltages V_(ind) ^(tot)=V_(A)+V_(B) in the inductors as afunction of the radius of the first inductor A. In this example, theradius of the second inductor B is kept constant and is 2.0 mm. It canbe noticed that the maximum of that upper curve corresponds to asymmetrical inductor configuration, in which the two inductors have thesame dimensions and are positioned at an equal distance from the rotor3.

The lower (thick) curve in FIG. 8 shows the sum of the induced voltagesaveraged over the pulse duration as a function of the radius of thefirst inductor A. It can now be noticed that the maximum valuecorresponds to an asymmetrical situation, in which the first and secondinductors are not identical. In this example, the external radius of thefirst inductor A is greater than then the external radius of the secondinductor B, as also shown in FIG. 6. This configuration thus minimisesthe difference V_(bat)−V _(ind) ^(tot), where V_(bat) is the supplyvoltage and V _(ind) ^(tot) is the average value of the sum of theinduced voltages in the two inductors over a drive pulse duration.

It is to be noted that in the above description, the peak inducedvoltage in the first inductor A was greater than the peak inducedvoltage in the second inductor B (see FIG. 4). Furthermore, in a givennormal rotation of the rotor, a given radius reaches first the secondinductor B (smaller induced voltage) and after that the first inductor A(greater induced voltage). In other words, in a normal rotation of therotor, a given magnet of the rotor is first aligned with the secondinductor and only after having rotated more (in this examplecounter-clockwise), it becomes aligned with the first inductor. If therotor were to be rotated in the opposite direction, then the crossingpoint of the induced voltages would be located timewise after the peakof the sum of the induced voltages. This would mean that the drive pulseto be generated would be located offset with respect to the centre ofthe peak of the sum of the induced voltages, what is contrary to thepresent invention.

Furthermore, in the above example, the pulse was generated substantiallyimmediately after the crossing of the induced voltages was detected.However, it is possible to trigger the pulse after a given delay. Forthis purpose, a timer may be used such that the time starts to run whena voltage crossing has been detected and a pulse is triggered once thetimer has expired. The delay may then be taken into account whendesigning the inductors. More specifically, the greater the delay, themore the two inductors differ from each other.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive, theinvention being not limited to the disclosed embodiment. Otherembodiments and variants are understood, and can be achieved by thoseskilled in the art when carrying out the claimed invention, based on astudy of the drawings, the disclosure and the appended claims.

1. A direct current electric motor comprising: a rotor equipped withpermanent magnets, the rotor being arranged to rotate continuously in adetermined rotation direction; a first stator inductor characterised byfirst inductor parameters; a second stator inductor characterised bysecond inductor parameters; a voltage supply unit for providing avoltage supply to the first and second stator inductors for driving therotor; an measurement unit for detecting time instants when a firstinduced voltage in the first stator inductor equals a second inducedvoltage in the second stator inductor; a control unit for controllingthe application of drive voltage pulses by the voltage supply unit tothe first and second stator inductors, wherein the rotor is arranged tofirst face the second stator inductor before facing the first statorinductor when being rotated in the determined rotation direction;wherein at least one of the second inductor parameters is different froma corresponding parameter of the first inductor parameters such that amaximum induced voltage in the first stator inductor is greater than amaximum induced voltage in the second stator inductor, and wherein thecontrol unit is arranged to trigger each drive voltage pulse after adetection, by the measurement unit, of an equal induced voltage in eachof the first and second stator inductors.
 2. The motor according toclaim 1, wherein the control unit is arranged to trigger the drivevoltage pulses each immediately after the detection of an equal inducedvoltage.
 3. The motor according to claim 1, wherein at least one of thesecond inductor parameters comprises at least one structural dimensionof the second inductor.
 4. The motor according to claim 1, wherein thefirst and second inductors are each formed by a coil, and wherein atleast one of the second inductor parameters comprises at least one ofthe following parameters: a coil wire diameter, a number of wire turns,a coil dimension and a radial coil position with respect to an axis ofrotation of the rotor.
 5. The motor according to claim 1, wherein afirst distance defined as a minimal distance between a centre of therotor and an outer edge of the first stator inductor substantiallyequals a second distance defined as a minimal distance between thecentre of the rotor and an outer edge of the second stator inductor. 6.The motor according to claim 5, wherein the first distance and thesecond distance are substantially equal to or greater than a thirddistance defined as a distance between the peripheries of the firststator inductor and the second stator inductor.
 7. The motor accordingto claim 1, wherein the control unit is arranged to apply the drivevoltage pulses so that the drive voltage pulses are substantiallycentred at an absolute maximum of a sum of the induced voltages in thefirst and second inductors.
 8. The motor according to claim 1, the firstand second inductors are at an angle ∝ relative to each other, the angle∝ being defined as the angle between a first imaginary line, passingthrough the axis of rotation of the rotor and the centre of the firstinductor, and a second imaginary line passing through the axis ofrotation of the rotor and the second inductor, and wherein the angle ∝is between 95° and 115°.
 9. The motor according to claim 1, wherein thefirst and second inductors are disc-shaped such that the externaldiameter of each one of the inductors is greater than the thickness ofeach one of the inductors.
 10. The motor according to claim 1, whereinthe first and second inductors are connected in a series configurationduring the drive voltage pulses.
 11. An electromechanical watchcomprising the motor according to claim
 1. 12. An electromechanicalwatch comprising the motor according to claim
 4. 13. A method ofoperating a direct current electric motor comprising: a rotor equippedwith permanent bipolar magnets, the rotor being arranged to rotatecontinuously in a determined rotation direction; a first stator inductorcharacterised by first inductor parameters; a second stator inductorcharacterised by second inductor parameters; a voltage supply unit forproviding a voltage supply to the first and second stator inductors fordriving the rotor; a measurement unit for detecting time instants when afirst induced voltage in the first stator inductor equals a secondinduced voltage in the second stator inductor, a control unit forcontrolling the application of drive voltage pulses by the voltagesupply unit to the first and second stator inductors, wherein the methodcomprises rotating the rotor in the determined rotation direction suchthat it first faces the second inductor before facing the firstinductor; wherein at least one of the second inductor parameters isdifferent from a corresponding parameter of the first inductorparameters such that a maximum induced voltage in the first statorinductor is greater than a maximum induced voltage in the second statorinductor, and wherein the control unit triggers each drive voltage pulseafter a detection, by the measurement unit, of an equal induced voltagein each of the first and second stator inductors.
 14. The methodaccording to claim 13, wherein the drive voltage pulses are triggeredimmediately after the detection of an equal induced voltage.
 15. Themethod according to claim 12, wherein the drive voltage pulses areapplied so that these drive voltage pulses are centred at an absolutemaximum of a sum of the induced voltages in the first and secondinductors.